The use of laser-generated electromagnetic optical energy has become ubiquitous in applications ranging in areas from commercial to national defense. An illustrative example of the former is the use of optical energy for industrial materials processing, including cutting, welding and surface treatment. End products resulting from such implementations come from such wide ranging business areas as automotive, aerospace, appliance and shipbuilding. More exotic applications can include rock drilling for mining and/or oil and gas exploration purposes. Directed energy or so-called “laser weapons” are finding increased acceptance in the defense community because of the position-sensitive lethality delivery. As lasers become smaller, more efficient and robust, it is easier to integrate them into ground-based, sea-based, airborne and space-borne paradigms.
Solid-state lasers (SSLs), in particular, have received renewed attention in recent years, especially in high average power (HAP) applications, where large energy delivery allows for implementation in military and/or industrial uses. Such lasers include a solid-state host material in either crystal or glass form that is doped with suitable rare-earth ions. These ions are optically pumped with light generated by another optical source, such as a semiconductor diode or high intensity flashlamp. After absorption of the pumping light, the ions re-emit the light into the optical resonator, creating a coherent light or laser output. Various types of SSLs used in HAP applications have lasing media formed in disk, rod, slab (i.e. “zigzag”) or other shapes, depending on the pumping mechanism and whether the output is designed to optimize output power, efficiency or beam propagation characteristics.
Although SSLs are effective for many purposes, they frequently present design challenges due to waste heat deposited into the gain medium by the optical pumping. The frequency of optical pumping light is higher than the output laser frequency. The difference in these frequencies represents electronic energy, which is coupled into phonon excitations in rare earth ion host materials and eventually couples to the thermal excitation in the solid, this being loosely characterized as waste heat. This heat can cause various types of thermo-mechanical and thermo-optical distortions in the laser, resulting in nonuniformities in the refractive properties of the gain medium. These fluctuations manifest themselves in macroscopic optical degradations, such as thermal lensing, mechanical stresses, depolarization and other undesirable effects. These effects could result in degradation in beam quality (BQ), reduced laser power and/or possible fracture of the SSL lasing medium. Other types of lasers are similarly responsive to thermo-mechanical distortions such as thermal lensing and/or depolarization.
Many SSLs and other lasers include one or more optical lenses to focus light emanating from the gain medium, to compensate for thermal lensing and for other purposes. To compensate for thermal lensing, refractive elements or non-infinite radii of curvature reflective surfaces will be included internally to the optical resonator. Lasers also frequently include one or more birefringence compensators such as a Faraday rotator to suppress unwanted modes propagating in the laser, to reduce the effects of thermal depolarization and thermally induced birefringence, and the like. Birefringence compensators will typically be composed of uniaxial or biaxial crystals with so-called optical activity. Optical activity is a material property in which the refractive index experienced by an electromagnetic beam is dependent on the polarization of the beam relative to an optical axis within the crystal structure. While the addition of lenses and birefringence compensators provides quantitative improvement in the laser performance, these elements unfortunately increase the mechanical complexity of the laser, and may lead to design difficulties due to the increased optical and mechanical instability of additional elements within the optical propagation path of the laser. More particularly, the additional optical elements makes the laser more susceptible to mechanical vibrations and subsequent mis-alignments, thereby increasing maintenance costs and/or degrading laser performance. The additional elements may also add undesirable weight and volume to the laser, further complicating opto-mechanical design.
It is therefore desirable to reduce the weight, volume and complexity of the laser by reducing the number of components, particularly within the optical propagation path of the laser. It is also desirable to create a laser with reduced complexity that is capable of high average power applications. Other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background discussion.